We present the first laboratory-in-the field experiment on the Italian North-South divide. Using a representative sample of the population, we measure whether regional disparities in ability to cooperate emerge even if differences in geography, institutions and criminal intrusion are silenced. We report that a behavioural gap in cooperation exists: Northern and Southern citizens react differently to the same incentives. Moreover, this gap cannot be accounted for by tolerance for risk, proxies of social capital and 'amoral familism'. At least a share of North-South disparities is likely to derive from persistent differences in social norms.